1. Field of the Invention
The present invention relates to a call admission control device and a call admission control method, and more specifically to a call admission control device and a call admission control method for controlling the admission of a call in a packet communication system in mobile communications.
2. Description of the Related Art
A mobile communication system performs a communication using finite resources (frequencies and power), and there is the upper limit to the communication capacity. Therefore, it is necessary to limit the number of mobile stations in a cell depending on the communication capacity. Practically, when a new mobile station starts communications in the cell, it is necessary to judge whether or not the new mobile station can start a communication in the cell. The control is referred to as call admission control. The status in which a new mobile station cannot start a communication in the cell, that is, the status in which the communication capacity is being used substantially 100%, is referred to as a capacity limit.
The conventional call admission control is a control method of, for example, allowing a new mobile station to start communications when the total number of mobile stations being communicating in the cell does not exceed a predetermined threshold value, and not allowing a new mobile station to start communications when the total number of mobile stations being communicating in the cell exceeds the predetermined threshold value.
Relating to the standardization of a third generation mobile communication system, what is called IMT-2000, the standard specifications related to the W-CDMA system and the CDMA 2000 system are prescribed respectively for the 3GPP (third-generation partnership project) and 3GPP2 (third-generation partnership project 2).
In the 3GPP, with the recent fast spread of the Internet, based on the prediction that high speed and large capacitance traffic by the download, etc. from a database and a Web site especially in the downlink, the specification of the “HSDPA (high speed downlink packet access)” as a high speed packet transmission system in the downlink direction is prescribed (for example, refer to 3GPP Tr25.848 v4.0.0).
As for the 3GPP2, from a similar point of view, the specification of the “1x-EVDO” as a high speed dedicated transmission system in the downlink direction is prescribed (for example, refer to 3GPP2 C.S0024 Rev.1.0.0). In the CDMA 2000 1xEV-DO, the DO means “Data Only”.
Further described below is the HSDPA.
The HSDPA is a system for performing communications by sharing one or more than two shared channel among a plurality of mobile stations, and a radio base station selects a mobile station which uses the shared channel from among the plurality of mobile stations for each TTI (time transmission interval, 2 ms for the HSDPA) for transmission of a packet. In this case, depending on a way of an occurrence of data traffic, there is a difference in number of mobile stations in a cell when a capacity limit is reached. For example, when the case where all mobile stations download data using an FTP (file transfer protocol) is compared with the case where all mobile stations perform Web browsing, the number of mobile stations in the cell when the capacity limit is reached is larger in the latter case. Because there are packets to be transmitted to the mobile stations all the time exist in downloading data using the FTP, while, in the case of the Web browsing, a user has reading time to browse Web pages and there is time when no packet to be transmitted to a mobile station exists. That is, in the case of the Web browsing, since there is a low frequency at which one mobile station uses a shared channel, more mobile stations can share one shared channel. As a result, in the HSDPA, the number of mobile stations that can be accumulated in a capacity limit fluctuates depending on the way of the occurrence of data traffic.
In the HSDPA, a modulation scheme of a radio channel, a number of codes and a coding rate are adopted depending on the radio channel quality between a mobile station and a radio base station (in the HSDPA, it is called an adaptive modulation and coding scheme), and the transmission data rate fluctuates depending on the radio channel quality (SIR: Signal-to-Interference power Ratio) between the mobile station and the radio base station. On the other hand, the radio channel quality (SIR) largely depends on the mode of a cell such as an outdoor environment and an indoor environment, an urban area and a suburb, etc. That is, in the HSDPA, the number of mobile stations that can be accumulated at the capacity limit fluctuates depending on the mode of a cell.
In the HSDPA, when the call admission control in the above-mentioned conventional method, that is, the call admission control based on the number of mobile stations currently performing communications in a cell, is performed, the number of mobile stations performing communications in the cell in a capacity limit is constant. Therefore, the transmission data rate of each mobile station in a capacity limit depends on the position of each mobile station or the mode in a cell. For example, since there is little interference from other cells in an indoor environment, and the radio channel quality is high, the transmission data rate of each mobile station in a capacity limit is high. However, since there is much interference in an outdoor environment, and the radio channel quality is not high, it is considered that the transmission data rate of each mobile station in a capacity limit is low. However, the transmission data rate of each mobile station in a capacity limit is to be determined by the serviceability of the communications provided using the HSDPA, and it is desired that the speed is independent of the way of the occurrence of data traffic and the form of cells.
JP2002-217956A (hereinafter referred to as patent document 1) describes the technology of controlling the admission of a new call by applying a call admission threshold value to the use status of resources.
As described above, there is a method of not admitting a call by a new mobile station when a total number of mobile stations performing communications in a corresponding cell exceeds a predetermined threshold value as a call admission control method in a communication system which transmits a packet to a plurality of mobile stations.
However, the above-mentioned conventional call admission control method has the disadvantage of not able to appropriately controlling admission of a call depending on the method of generating data traffic and the mode of a cell. That is, when the conventional call admission control method is used, there occurs the problem that the transmission data rate of a mobile station in a capacity limit depends on the method of generating data traffic and the mode of a cell. This problem cannot be solved by the technique according to the above Patent Document 1.